An illumination system of the type described in the opening paragraph is known as such. More specifically, such a system is disclosed in the granted patent U.S. Pat. No. 7,402,840 B2. This document teaches an illumination system having a semiconductor light emitting device (LED), a wavelength converting material and a filter material. The light emitting device is able to emit a first light with a first peak wavelength, which is absorbed by the wavelength converting material. This material (most often a luminescent material, such as an inorganic phosphor) is able to emit the absorbed first light as a second light with a second peak wavelength. The known illumination system further contains a filter, which is able to transmit the second light, but which reflects or absorbs a portion of the first light. With the known system, it is possible to produce light having high color purity. The use of the filter especially prevents that the first (or primary) light generated by the LED escapes from the system. Thus the only light emitted by the system is high color purity second (or secondary) light being emitted by the wavelength converting material.
Illumination systems of the described design often show the disadvantage that the bandwidth of their emission spectra is relatively large. There is a general need for illumination systems comprising one or more phosphor-based LEDs, which emit radiation of a relatively small spectrum band. This holds especially for systems that emit light in the red region of the electromagnetic spectrum, preferably in the region around 600-630 nm.